The past couple of months, my Ubuntu 14.04 machine will freeze up. When it happens, none of the options I've read about to get to a terminal or restart gracefully work. The only thing I can do is a hard reboot. I've run a memory test, and everything checks out. I'm not sure what else I can check. I'm considering replacing the graphics card, as it is an older ATI Radeon HD 4850. It seems to be running just under 80 degrees. However, I don't really want to put the money into replacing it, if that's not the issue. Does anyone have any other suggestions as to how I can diagnose this?